The following references are related to the art of “transparent and conductive electrodes”:    1. L. Hu, D. S. Hecht, and G. Gruner, “Percolation in Transparent and Conducting Carbon Nanotube Networks,” Nano Letters, 2004, 4, 2513-2517.    2. Z. Wu, et al. “Transparent, Conductive Carbon Nanotube Films,” Science 27 Aug. 2004: Vol. 305 no. 5688 pp. 1273-1276.    3. H. G. Park, et al., “Transparent Conductive Single Wall Carbon Nanotube Network Films for Liquid Crystal Displays, ECS Solid State Lett. 2 Oct. 2012: R31-R33.    4. Jung-Yong Lee, Stephen T. Connor, Yi Cui, and Peter Peumans, “Solution-Processed Metal Nanowire Mesh Transparent Electrodes,” Nano Letters, 2008, 8 (2), pp 689-692.    5. S. De, et al., “Silver Nanowire Networks as Flexible, Transparent, Conducting Films: Extremely High DC to Optical Conductivity Ratios,” ACS Nano, 2009, 3, 1767-1774.    6. Ting-Gang Chen, et al., “Flexible Silver Nanowire Meshes for High-Efficiency Microtextured Organic-Silicon Hybrid Photovoltaics,” ACS Applied Materials & Interfaces, 2012, 4 (12), 6857-6864.    7. Taegeon Kim, et al., “Electrostatic Spray Deposition of Highly Transparent Silver Nanowire Electrode on Flexible Substrate, ACS Appl. Mater. Interfaces, Article ASAP; DOI: 10.1021/am3023543.    8. Y. Ahn, Y. Jeong, and Y. Lee, “Improved Thermal Oxidation Stability of Solution-Processable Silver Nanowire Transparent Electrode by Reduced Graphene Oxide,” ACS Applied Materials & Interfaces, 2012, 4 (12), 6410-6414.    9. G. Gruner, L. Hu, and D. Hecht, “Graphene Film as Transparent and Electrically Conductive Material,” US Patent Pub. No. 2007/0284557 (Dec. 13, 2007).    10. L. Hu, et al., “Touch Screen Devices Employing Nanostructure Network,” US Patent Pub. No. 2008/0048996 (Feb. 28, 2008).    11. G. Gruner, et al.; “Graphene Film as Transparent and Electrically Conductive Material,” US Patent Pub. No. 2009/0017211 (Jan. 15, 2009).    12. G. Eda, et al., “Large-Area Ultrathin Films of Reduced Graphene Oxide as a Transparent and Flexible Electronic Material. Nature Nanotechnology, 2008, 3, 270-274.    13. X. Wang, L. Zhi, and K. Mullen, “Transparent, Conductive Graphene Electrodes for Dye-Sensitized Solar Cells. Nano Letters, 2008, 8, 323.    14. J. B. Wu, et al., “Organic Light-Emitting Diodes on Solution-Processed Graphene Transparent Electrodes,” ACS Nano 2009, 4, 43-48.    15. S. De and J. N. Coleman, “Are There Fundamental Limitations on the Sheet Resistance and Transparence of Thin Graphene Films?” ACS Nano, 2010 May 25; 4(5), pp. 2713-20.    16. K. S. Kim, et al. “Large-Scale Pattern Growth of Graphene Films for Stretchable Transparent Electrodes,” Nature, 2009, 457, 706-710.    17. X. S. Li, et al. “Transfer of Large-Area Graphene Films for High-Performance Transparent Conductive Electrodes,” Nano Letters, 2009, 9, 4359-4363.    18. A. Reina, et al. “Large Area, Few-Layer Graphene Films on Arbitrary Substrates by Chemical Vapor Deposition,” Nano Letters, 2009, 9, 30-35.    19. Sukang Bae, et al., “Roll-to-roll production of 30-inch graphene films for transparent electrodes,” Nature Nanotechnology, Vol. 5, August 2010, 574-578.    20. V. C. Tung, et al. “Low-Temperature Solution Processing of Graphene-Carbon Nanotube Hybrid Materials for High-Performance Transparent Conductors” Nano Letters, 2009, 9, 1949-1955.    21. I. N. Kholmanov, et al. “Improved Electrical Conductivity of Graphene Films Integrated with Metal Nanowires,” Nano Letters, 2012, 12 (11), pp 5679-5683.
Optically transparent and electrically conductive electrodes are widely implemented in optoelectronic devices, such as photovoltaic (PV) or solar cells, light-emitting diodes, organic photo-detectors, and various display devices. For use in these applications, the electrode materials must exhibit both exceptionally high optical transmittance and low sheet resistance (or high electrical conductivity). More commonly used transparent and conductive oxides (TCO) for the electrodes in these devices include (a) indium tin oxide (ITO), which is used for organic solar cells and light-emitting diodes, and (b) Al-doped ZnO, which is used in amorphous solar cells. There are some alternatives to these TCO that are being considered, such as single-walled carbon nanotubes (CNT), graphene, and metal or metal nanowires (NW).
Discrete carbon nanotubes may be used to form a thin film of highly porous network (or mesh) of electron-conducting paths on an optically transparent substrate, such as glass or polymer (e.g., polyethylene terephthalate, PET or polycarbonate). The empty spaces between nanotubes allow for light transmission and the physical contacts between nanotubes form the required conducting paths [Refs. 1-3]. However, there are several major issues associated with the use of CNTs for making a transparent conductive electrode (TCE). For instance, a higher CNT content leads to a higher conductivity, but lower transmittance due to a lower amount of empty spaces. Further, the sheet resistances of CNT-based electrodes are dominated by the large CNT junction resistances due to the mixed carbon nanotube varieties, with ⅓ being metallic and ⅔ semiconducting. As a result, a typical sheet resistance of CNT networks on a plastic substrate is 200-1,000 ohms/square (Ω/□) at an optical transmittance of 80-90%. The relatively high sheet resistance, compared with the approximately 10-50 ohms/square of high-end ITO on a plastic substrate, is far from being adequate for the practical application of transparent CNT electrodes in current-based devices, such as organic light emitting diodes and solar cells. Furthermore, an optical transmittance of >90% is generally required for these devices. Even for voltage-driven devices, such as capacitive touch screens, electro-wetting displays, and liquid crystal displays, a relatively low sheet resistance is highly desirable.
Metal nanowire mesh-based conductive and transparent films are also being considered as a potential replacement for ITO [Refs. 4-8]. However, metal nanowires also suffer from the same problems as CNTs. For instance, although individual metal nanowires (e.g. Ag nanowires) can have a high electrical conductivity, the contact resistance between metal nanowires can be significant. Additionally, although Ag nanowire films can show good optical and electrical performance, it has been difficult to make Ag nanowires into a free-standing thin film or a thin film of structural integrity coated on a substrate. In particular, Ag nanowire films that are deposited on a plastic substrate exhibit unsatisfactory flexibility and mechanical stability in that the nanowires can easily come off. Also, the surface smoothness is poor (surface roughness being too large).
Furthermore, all metal nanowires still have a long-term stability issue, making them unacceptable for practical use. When Ag nanowire films are exposed to air and water, Ag nanowires can be easily oxidized, leading to sharp increase in sheet resistance and hazeness of the films. Ahn, et al [Ref. 8] disclosed the deposition of a reduced graphene oxide (RGO) layer or multiple RGO layers to a pre-fabricated Ag nanowire layer. The intent was to protect the underlying Ag nanowire film, but this approach can introduce additional issues to the film, e.g. significantly reduced optical transmittance by carrying out multiple coating passes and increased sheet resistance (when the Ag nanowire film was coated with more than 3 passes).
Graphene is yet another potential alternative to ITO. An isolated plane of carbon atoms organized in a hexagonal lattice is commonly referred to as a single-layer graphene sheet. Few-layer graphene refers to a stack of up to 5-10 planes of hexagonal carbon atoms bonded along the thickness direction with van der Waals forces. The generally good optical transparency and good electrical conductivity of graphene have motivated researchers to investigate graphene films for transparent and conductive electrode (TCE) applications [Refs. 9-21].
For instance, Gruner et al [Refs. 9-11] suggested a transparent and conductive film comprising at least one network of “graphene flakes,” which are actually very thick graphite flakes. A suspension of graphite flakes in a solvent was deposited onto a transparent glass, allowing isolated graphite flakes to somehow overlap one another to form a mesh (e.g. FIG. 1 of Ref. 9 and FIG. 1 of Ref. 11). The empty spaces between graphite flakes permit the light to pass through. However, these films typically exhibit a sheet resistance as high as 50 kOhm/square (5×104Ω/□) at 50% transparency. The low transparency is a result of using thick graphite flakes, not graphene sheets. Gruner et al then attempted to improve the film performance by combining carbon nanotubes and graphite flakes to form an interpenetrating network of conductive pathways (e.g. FIG. 2 of Ref. 9 and FIG. 2 of Ref. 11). Unfortunately, the interpenetrating network of graphite flakes and carbon nanotubes lead to a film that is only 80% transparent at 2 kOhms/square or 65% transparent at 1 kOhms/square (e.g., paragraph [0026] in both Ref. 9 and Ref. 11). These values are absolutely unacceptable to the TCE industry.
In a graphene film made by metal-catalyzed chemical vapor deposition (CVD), each graphene plane loses 2.3-2.7% of the optical transmittance and, hence, a five-layer graphene sheet or a film with five single-layer graphene sheets stacked together along the thickness direction would likely have optical transmittance lower than 90%. Unfortunately, single-layer or few layer graphene films, albeit optically transparent, have a relatively high sheet resistance, typically 3×102-105 Ohms/square (or 0.3-100 kΩ/□). The sheet resistance is decreased when the number of graphene planes in a film increases. In other words, there is an inherent tradeoff between optical transparency and sheet resistance of graphene films: thicker films decrease not only the film sheet resistance but also the optical transparency.
A recent study [Ref. 19] has demonstrated that single-layer CVD graphene films can have sheet resistances as low as ˜125Ω/□ with 97.4% optical transmittance. However, the sheet resistance is still lower than desirable for certain applications. The authors further used layer-by-layer stacking to fabricate a doped four-layer film that shows sheet resistance at values as low as ˜30Ω/□ at ˜90% transparency, which is comparable to those of certain ITO grades. However, the layer-by-layer procedure is not amenable to mass production of transparent conductive electrodes for practical uses. Doping also adds an extra level of complexity to an already highly complex and challenging process that requires a tight vacuum or atmosphere control. The CVD process and equipment are notoriously expensive. Strong and urgent needs exist for more reliable and lower-cost processes and/or TCE materials that exhibit outstanding performance (e.g. sheet resistance <30-40Ω/□, yet still maintaining a transparency no less than 90%).
Since both graphene and carbon nanotube (CNT) have carbon atoms as the primary element, it is appropriate to briefly discuss carbon-based materials at this juncture. Carbon is known to have five unique crystalline structures, including diamond, fullerene (0-D nano graphitic material), carbon nano-tube or carbon nano-fiber (1-D nano graphitic material), graphene (2-D nano graphitic material), and graphite (3-D graphitic material). The carbon nano-tube (CNT) refers to a tubular structure grown with a single wall or multi-wall. Carbon nano-tubes (CNTs) and carbon nano-fibers (CNFs) have a diameter on the order of a few nanometers to a few hundred nanometers. Their longitudinal, hollow structures impart unique mechanical, electrical and chemical properties to the material. The CNT or CNF is a one-dimensional nano carbon or 1-D nano graphite material.
Bulk natural flake graphite is a 3-D graphitic material with each particle being composed of multiple grains (a grain being a graphite single crystal or crystallite) with grain boundaries (amorphous or defect zones) demarcating neighboring graphite single crystals. Each grain is composed of multiple graphene planes that are oriented parallel to one another. A graphene plane in a graphite crystallite is composed of carbon atoms occupying a two-dimensional, hexagonal lattice. In a given grain or single crystal, the graphene planes are stacked and bonded via van der Waal forces in the crystallographic c-direction (perpendicular to the graphene plane or basal plane). Although all the graphene planes in one grain are parallel to one another, typically the graphene planes in one grain and the graphene planes in an adjacent grain are different in orientation. In other words, the orientations of the various grains in a graphite particle typically differ from one grain to another.
The constituent graphene planes of a graphite crystallite can be exfoliated and extracted (or isolated) to obtain individual graphene sheets of carbon atoms provided the inter-planar van der Waals forces can be overcome. An isolated, individual graphene sheet of carbon atoms is commonly referred to as single-layer graphene. A stack of multiple graphene planes bonded through van der Waals forces in the thickness direction with an inter-graphene plane spacing of 0.3354 nm is commonly referred to as a multi-layer graphene. A multi-layer graphene platelet has up to 300 layers of graphene planes (<100 nm in thickness). When the platelet has up to 5-10 graphene planes, it is commonly referred to as “few-layer graphene” in the scientific community. Single-layer graphene and multi-layer graphene sheets are collectively called “nano graphene platelets” (NGPs). Graphene sheets/platelets (NGPs) are a new class of carbon nano material (a 2-D nano carbon) that is distinct from the 0-D fullerene, the 1-D CNT, and the 3-D graphite.
Our research group pioneered the development of graphene materials and related production processes as early as 2002: (1) B. Z. Jang and W. C. Huang, “Nano-scaled Graphene Plates,” U.S. Pat. No. 7,071,258 (Jul. 4, 2006), application submitted on Oct. 21, 2012; (2) B. Z. Jang, et al. “Process for Producing Nano-scaled Graphene Plates,” U.S. patent application Ser. No. 10/858,814 (Jun. 3, 2004); and (3) B. Z. Jang, A. Zhamu, and J. Guo, “Process for Producing Nano-scaled Platelets and Nanocomposites,” U.S. patent application Ser. No. 11/509,424 (Aug. 25, 2006).
It may be noted that NGPs include discrete sheets/platelets of single-layer and multi-layer pristine graphene, graphene oxide, or reduced graphene oxide with different oxygen contents. Pristine graphene has essentially 0% oxygen. Graphene oxide (GO) has 0.01%-46% by weight of oxygen and reduced graphene oxide (RGO) has 0.01%-2.0% by weight of oxygen. In other words, RGO is a type of GO having lower but non-zero oxygen content. Additionally, both GO and RGO contain a high population of edge- and surface-borne chemical groups, vacancies, oxidative traps, and other types of defects, and both GO and RGO contain oxygen and other non-carbon elements, e.g. hydrogen [Ref. 14; J. B. Wu, et al]. In contrast, the pristine graphene sheets are practically defect-free and contain no oxygen. Hence, GO and RGO are commonly considered in the scientific community as a class of 2-D nano material that is fundamentally different and distinct from pristine graphene.
It may be further noted that CVD graphene films, although relatively oxygen-free, tend to contain a significant amount of other non-carbon elements, such as hydrogen and nitrogen. The CVD graphene is polycrystalline and contains many defects, e.g., grain boundaries, line defects, vacancies, and other lattice defects, such as those many carbon atoms configured in pentagons, heptagons, or octagons, as opposed to the normal hexagon. These defects impede the flow of electrons and phonons. For these reasons, the CVD graphene is not considered as pristine graphene in the scientific community.
Pristine graphene can be produced by direct ultrasonication or liquid phase production, supercritical fluid exfoliation, direct solvent dissolution, alkali metal intercalation and water-induced explosion of natural graphite particles, or more expensive epitaxial growth. Pristine graphene is normally single-grain or single-crystalline, having no grain boundaries. Further, pristine graphene essentially does not contain oxygen or hydrogen. However, if so desired, the pristine graphene can be optionally doped with a chemical species, such as boron or nitrogen, to modify its electronic and optical behavior in a controlled manner.
A hybrid material containing both graphene oxide and CNT was formed into a thin film by Tung et al [Ref. 20], but the film does not exhibit a satisfactory balance of optical transparency and electrical conductivity. The highest performance film shows optical transmittance of 92%, but this is achieved at an unacceptable sheet resistance of 636Ω/□. The film with the lowest sheet resistance (240Ω/□ with un-doped RGO) shows 60% optical transmittance, which is not useful at all. The graphene component was prepared from heavily oxidized graphite, which was then intensely reduced with hydrazine.
Another hybrid material, containing non-pristine graphene (obtained by CVD) and silver nanowires, was formed into a film [Ref. 22]. Again, CVD-grown graphene is a polycrystalline material (not single-crystalline and not pristine) with many topological defects, such as non-hexagonal carbon atoms, vacancies, dislocations, and grain boundaries. Grain boundaries in graphene are line defects at the interfaces between two domains with different crystallographic orientations. Due to the processing conditions inherent to the CVD process, the CVD graphene also contains non-carbon elements (e.g. hydrogen) and non-hexagonal carbon atoms. All these characteristics (defects and impurities) can significantly impede the transport of electrons and phonons in CVD graphene films. Even with the help from silver nanowires, the best CVD graphene-AgNW hybrid film exhibits a sheet resistance value that is still far away from what can be theoretically achieved with graphene alone [Ref. 22]. Besides, CVD processes are slow and expensive.
As discussed above, the CNT mesh, metal nanowire mesh, CVD graphene film, GO film (including RGO film), CNT-graphite flake mesh, CNT-graphene oxide (GO) hybrid, and RGO-protected Ag nanowire mesh have been proposed to serve as a transparent and conductive electrode, but none has met the stringent combined requirements of transparency, conductivity, oxidation resistance or long-term stability, mechanical integrity and flexibility, surface quality, chemical purity, process ease, and low cost.
Thus, it is an object of the present invention to provide a graphene oxide gel-derived hybrid film containing metal nanowires that meets most or all of the aforementioned requirements. The graphene oxide gel unexpectedly acts to remove the otherwise readily formed metal oxide or other metal compounds that are highly insulating and lead to an extremely high contact resistance.
It is another object of the present invention to provide a process for producing a graphene oxide gel-derived hybrid film (containing graphene oxide or reduced graphene oxide) that is a variable alternative to ITO. Surprisingly, this process inherently removes metal oxide or metal compounds that automatically form on the surface of metal nanowires (e.g. Ag and Cu nanowires) when exposed to open air. This process also enables the coverage of metal nanowires (after removal of metal oxide or other metal compounds) with the RGO film, preventing the surface of metal nanowires from re-exposing to open air or any other oxidizing environment.